This invention relates to stabilizing a freight brake emergency valve device, particularly against transient pressure fluctuations that are known to cause undesired emergency brake applications, without consequent loss of emergency sensitivity,
Undesired emergency brake applications have long been a source of concern to the railroads. An in-depth study of this problem has identified the dynamic effects of slack run-in and run-out as being the main source of pressure transients in the train air line during over-the-road operation. Tests have shown that during slack action, bending of the trainline hose between cars occurs, which produces momentary volumetric changes in the trainline and consequent pressure "spikes" or pulses. In addition, the trainline air mass follows Newton's laws of motion, in that a general pressure drop of approximately 0.4 psi occurs at the rear of a train during slack run-in, with a similar drop in pressure at the head of the train occurring during slack run-out. The magnitude of these pressure fluctuations varies with the intensity of the slack action and other train parameters. When combined with quick service activity during service brake applications, these transient pressure fluctuations may generate a momentary, localized pressure reduction in the trainline at a rate that either exceeds, or so closely approaches an emergency rate, that a particularly sensitive control valve emergency piston may respond to trigger an undesired emergency brake application, as now explained.
The emergency valve in the well-known, industry standard, ABD/ABDW type control valve devices includes a piston having a slide valve that makes a "breather" port connection via which quick action chamber pressure on one side of the emergency piston vents to atmosphere via a "breather" choke, when the trainline brake pipe pressure effective on the opposite side of the emergency piston is reduced. This "breather" choke is selected to establish a maximum rate at which the fixed volume quick action chamber pressure is capable of venting. By setting this rate in accordance with a threshold rate of reduction of brake pipe pressure, above which it is desired to initiate an emergency brake application, a pressure differential is prevented from developing across the emergency piston for a duration of time sufficient to force the emergency piston to emergency position during service rates of reduction of brake pipe pressure. Only when the reduction of brake pipe pressure exceeds and sustains this threshold rate should a pressure differential be developed across the emergency piston sufficient to force the emergency piston to application position, to initiate an emergency brake application.